The invention relates to a method of storing signals having a course over time in an implantable medical device, and/or transmitting the signals out of the device and out of the patient's body, with the course over time being detected in the device, particularly at predetermined time intervals with a constant sampling interval, and a quantity of signal samples being obtained therefrom, of which a partial quantity is stored and/or transmitted as the result of a selection made with a predetermined selection criterion, and an apparatus for executing the method.
The detection and evaluation of physiologically-relevant signals in the body with suitable detection or monitoring devices are in step with the continuously-expanding use of implantable medical devices such as cardiac pacemakers or defibrillators. The operation of the implantable device is controlled on the basis of the results of, for example, the detection and evaluation of the course over time of cardiac-action potentials or the interior cardiac pressure or volumetric impedance measurements, etc. In addition to, or independently of, the first-mentioned function, the data can be used for a separate medical diagnosis and/or analysis of the operating state of the device itself.
The relevant signals typically result in analog form as high-resolution measurement curves, and, for many applications, must be registered over a relatively lengthy time period and kept ready for a comparative evaluation. In conventional handling, this requires storage of a large quantity of data in the implantable device and/or transmission of the data to the outside, and, finally, appropriate data processing. The rapid advances in the production of cost-effective semiconductor memories having very large storage capacities, and in processor technology, have significantly increased the relevant possibilities in implantable devices; nevertheless, it is desirable to limit the quantity of data in order to reduce costs and effect the lowest-current operation possible. This applies particularly to devices in which it is desirable to detect and handle numerous types of relevant data over periods of days and weeks, for example for automatic pacemaker/cardioverters.
One possibility known from a plurality of technical applications—particularly communication and measuring techniques—is data compression. Numerous data compression methods are described in detail in the relevant literature.
These techniques have not yet been able to gain a foothold in the field of storage and transmission of physiological data, particularly in implantable devices, partly because of the high cost of the device technology and partly because of the specifics of the signals occurring here and the information to be obtained over the course of the signal processing. Thus, as before, in this field the resulting data are typically stored in digitized form, without compression.
This is the path followed by an EKG monitor according to EP 0 512 667 A1, which uses a diskette or similar data carrier as a storage medium.
EP 0 149 094 B1 describes a non-implantable recording system for the time-compromised registration of EKG wave forms. The key factor of this system is simply the reduction of the resolution capability of a conventional registration to recording paper, which is effected here by a thermo-line printer. The system only serves in a first screening as the precursor of the later, more precise detection of arrhythmia states, and is not suited for obtaining precise diagnostic information, and is especially ill-suited for controlling implanted devices.
An EKG data compression for storing EKG data in a pacemaker memory is described in EP 0 263 599 A2. This compression is based on the condition that the EKG signal is constant (lies near the zero line) for the majority of a cardiac cycle, so no signal value needs to be stored for these times. Therefore, the time span between predetermined changes in the EKG signal is stored, so the method represents a time-related variation of the known delta modulation. Turning points in the signal course are characterized by additional storage of a flag.
If this method is intended to effect a resolution that permits the reliable, autonomous control of a combination anti-arrhythmia device, a large amount of storage space is required for storing the precise times at which the signal changes occur, and the time-data conditioning for the actual (particularly comparative) evaluation requires a substantial outlay.
U.S. Pat. No. 5,312,446 describes an implantable medical device having a telemetry device that includes a storage of detected physiological signals with—selectively, multiple-stage—data compression. The compression process is based on step-wise separation of a few significant measuring points based on a comparison of the absolute measured values at consecutive sampling points. The (compromised) data are stored in analog form, which requires the provision of a special memory.